(1) Field of the Art
This invention relates to a thermally responsive switch having a thermally responsive element enclosed in a hermetically sealed vessel, said thermally responsive element being warped for snap action in response to ambient temperature.
A thermally responsive switch of this type thus far introduced, comprises a fixed contact disposed in a hermetically sealed vessel and a movable contact secured to a thermally responsive disk disposed in said vessel, with the disk being of bi- or tri-metallic element. Said switch is, for example, embedded into the winding of an electric motor with the disk in series with the winding. At the time of overloaded condition, the disk snaps to separate the movable contact from the fixed contact so as to interrupt the motor circuit for protection in response to the temperature rise owing to abnormally increased current flowing through the motor.
In so doing, said fixed contact support is in cantilever relationship with the vessel. Such is the construction that the fixed contact support has a tendency to bounce when the movable contact is brought into engagement with the fixed contact upon the moving-back action of the disk. The bounce thus induced allows both the contacts to be exposed to welding force caused by arcing. As a result, the contacts suffer from undesirably large quantity of consumption loss.
Further, it is important to insure a short response time for the disk to snap when its temperature reaches a predetermined value in protecting appliances to be protected against overcurrency.
In addition, it is necessary to maintain uniform response time in spite of having a means to adjust ultimate trip current value.